Reciprocable devices typically include a reciprocable member which reciprocates to perform a useful function, such as pumping a flowable material, compressing a gas, metering a fluid or providing a reciprocating output for other purposes. A driving fluid under pressure is commonly used to reciprocate the reciprocable member. For example, the reciprocable member may be a piston having first and second faces which are alternately exposable to driving fluid under pressure and to exhaust. A valve system is provided for controlling the exposure of the piston faces to the pressurized driving fluid and to exhaust. In order for the valve system to perform its function, the valve system typically includes one or more valve elements which must be moved periodically from one position to another to bring about reciprocation of the piston. One problem has been to provide a simple, reliable and inexpensive mechanism for repeatedly operating the valve system.
One way to operate the valve system is to drive the valve elements with energy from the piston as the piston nears the end of its stroke. Although this approach is theoretically sound, in practice, the piston will stall if the valve system is not completely operated before the piston stops. In this event, the switching of the valve system cannot occur, and the reciprocable device stalls.